The invention is based on a state of the art such as shown in German patent publication 2,307,550. In accordance therewith the shifting characteristics of a speed-change transmission connected to the output of a hydrostatic transmission are improved in that for correlation of the rotary speeds of the clutches to be engaged and disengaged in the speed-change transmission, the hydrostatic transmission at the beginning of a speed change is operated so that its output speed follows the rotary speed to be changed by a positive or negative increment. Thus the rotary speed differentials registered by the tachometers at the clutches to be engaged and disengaged are used to control a servovalve which itself operates the positioning apparatus.
In the power regulator of German patent publication 2,307,550 a particular positioning piston engages directly on the control rod of the power regulator and thereby also on the positioning rod extending from the regulator to the variableoutput hydraulic pump. This gives two disadvantages: First of all relatively large positioning forces are required by the heavy-duty and normally physically relatively large control element, and secondly during the speed-change operation pressure surges in the form of positive and negative pressure peaks occur which are effective over the operating pressure on the regulator and there produce pulses which interfere with the positioning. This therefore has a bad effect on the quality of the shifting operation.